Superconductive conjugate photoconductive substances of the Bi-SrCa(LaY)-Cu-O system, a method for producing the same and superconductive optoelectronic devices using the same

ABSTRACT

The disclosed substance has a composition of a general chemical formula of 
     
         Bi.sub.2 -(Sr.sub.2 Ca.sub.1).sub.1-x (La.sub.2 Y.sub.1).sub.x -Cu.sub.y 
    
      -O z , 
     where 0.4≦x≦1, y=2 and z=9-10.5, wherein said substance is an insulator or a semiconductor in the dark, and has a photoconductivity Q(λ,T) in conjugate with superconductivity of a superconductor of an adjacent component of the Bi-SrCa-LaY-Cu-O system at and below a critical temperature (T) of less than 105-115K and below 65-85K at photoexcitation in an optical wavelength range (λ) of 420-670 nm. The present invention relates to a method for producing the same and a superconductive optoelectronic device by using the same. The present invention also relates to an organized integration of the element or device into an apparatus to further develop a new field of &#34;Superconductive Optoelectronics.&#34;

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a substance of superconductiveconjugate photoconductivity in parallel to superconductivity in acomposition range outside the superconductive composition region withinthe Bi-SrCa(LaY)-Cu-O oxide system and a method for producing the sameand a superconductive optoelectronic device with the same.

Here, I define "Superconductive-Conjugate Photoconductivity" to be asubstantially new type of large photoconductivity in basic substances orhost insulators which emerges in several steps with decreasingtemperature in accordance or correspondence with the criticaltemperatures of superconductivity in relevant conductive substances, allbased on the discoveries and inventions disclosed by the presentinventor in that "Photoconductivity" and "Superconductivity" areconjugate with each other in a certain group of oxide superconductors.

2. Related Art Statement

The present inventor has presented series of substances havingphotoconductivity as the substances close to but outside ofsuperconductive region in the prior art, and has already filed patentapplications related to substances in the Y_(3-x) Ba_(x) -Cu_(y) -O_(z)oxide system of superconductive photoconductivity (Japanese PatentApplication Laid-open No. Hei-1(1989)-20158), to substance in the La₂-Cu₁ -O_(z) system of superconductive photoconductivity (Japanese PatentApplication Laid-open No. Hei-1(1989)-201059), to substance in the Ba₁-Pb_(1-x) -Bi_(x) -O_(z) oxide system of superconductivephotoconductivity and a method for producing the same (Japanese PatentApplication Laid-open No. Hei-2(1990)-51423) and to substance in theCa.sub.(x-x) -Sr_(x) -Bi.sub.(Y-y) -Cu_(y) -O_(z) oxide system ofsuperconductive photoconductivity and a method for producing the same(Japanese Patent Application Laid-open No. Hei-2(1990)-51424).

Before 1986, superconductive materials have signified essentially metalsand alloys thereof. However, recent oxide high temperaturesuperconductors (such as the Y-Ba-Cu-O oxide superconductor) areoriginally insulators or semiconductor and have been doped by using alarge amount of additional elements (such as Ba, Sr) for the purpose ofincreasing hole density and improving the critical temperature.Therefore, experiments of optical properties in the vicinity of theiroptically visible range were mainly limited to measurements of opticalreflection or scattering by reflecting metallic properties thereof.

An incident light reflects or scatters on the surface of superconductor,but never enters into a superconductor, so that superconductivity andoptical properties such as absorption have been usually considered to beirrelevant, except reflection and scattering of light, in domesticscientific societies and international conference abroad.

The reason thereof is because superconductivity are considered to beincompatible physical properties with absorption and photoconductivityand the stability of a superconductor is broken by irradiating light inthe wavelength range of shorter than those relevant to the gap energy ofthe BCS theory. However, there exist reasonably clear correlationsbetween photoconductivity in insulator and superconductivity in theoxide materials such as the Y-Ba-Cu-O, La-Cu-O, Ba-Pb-Bi-O systems ofoxide material and the like. Therefore, if a substance having either oneor both of deeply correlated superconductivity and photoconductivity isobtained, it becomes possible to utilize it to compose devices such asan optically controllable Josephson element or a superconductivephototransistor and the like and eventually to manufacture apparatushaving both properties of machinery and tools of "superconductivecomputer" based on the presently pursued Josephson element and of"optical computer" proposed by optoelectronics, that is,"superconductive optical computer" and the like.

SUMMARY OF THE INVENTION

The present invention is based on the discovery of photoconductivesubstances exhibiting a normally unforeseeable photoconductivephenomenon conjugate with superconductivity by performing an experimentof optical properties, particularly high speed pulse photoconductivity,of substance close to but outside the critical composition region ofsuperconductive substance.

In a method for producing the photoconductive material according to thepresent invention, photoconductive substance of the Bi₂ -(Sr₂ Ca₁)_(1-x)(La₂ Y₁)_(x) -Cu_(y) -O_(z) oxide system having photoconductivityconjugate with superconductivity of the Bi-SrCa(LaY)-Cu-O system (x=0)superconductor can be obtained by controlling a composition ratio x, yand z, thereafter heat treating to select a composition range to be y=2,and approximating to x=1 in 0.4≦x≦1 (preferably 0.5≦x≦1) or by coolingextremely quickly.

An object of the present invention is to provide a superconductiveconjugate photoconductive substance having photoconductivity Q(λ,T) at atemperature (T) of less than a critical temperature of correspondingsuperconductor (i.e. less than 105-115K and less than 65-85K) atphotoexcitation of a specified wavelength (λ) range of 420-670 nm.

The present invention relates to a superconductive conjugatephotoconductive substance consisting of a composition having a generalchemical formula

    Bi.sub.2 -(Sr.sub.2 Ca.sub.1).sub.1-x (La.sub.2 Y.sub.1).sub.x -Cu.sub.y -O.sub.z

where 0.4≦x≦1, y=2 and z=9-10.5, said substance is an insulator or asemiconductor in the dark, having photoconductivity Q(λ,T) at a criticaltemperature (T) of less than 105-115K and less than 65-85K, andphotoexcitation of an optical wavelength (λ) region of 420-670 nm.

Another object of the present invention is to provide a method forproducing superconductive conjugate photoconductive substance by heatinga starting material consisting of a composition having a generalchemical formula

    Bi.sub.2 -(Sr.sub.2 Ca.sub.1).sub.1-x (La.sub.2 Y.sub.1).sub.x -Cu.sub.y -O.sub.z

where 0.4≦x≦1, y=2 and z=9-10.5, at a temperature of 800°-840° C. forproducing a solid phase reaction for 8-15 hours, annealing for 8-15hours, forming with pressure, thereafter secondarily sintering at900°-940° C. for 8-15 hours, annealing at a cooling rate of 100°-150°C./H, and obtaining photoconductive substance conjugate withsuperconductivity.

Further object is to point out that the obtained substance can beexpected to be applied to "superconductive optoelectronics" as anindustrially applicable field.

The reason why the substance of the present invention is limited to thecomposition having the general chemical formula is becausesuperconductive conjugate photoconductive substance having thesuperconductive conjugate temperature dependences and the specifieddependences at photoexcitation wavelength even within an insulativecomposition range can be obtained as substantially shown in embodimentonly when the substance within this composition range is heated at atemperature of about 800°-840° C. for producing a solid phase reactionfor 8-15 hours, annealed for 8-15 hours, formed with pressure,thereafter secondarily sintered at 900°-940° C. for 8-15 hours, andannealed at a cooling rate of 100°-150° C./H.

The reason of limiting each condition of the method for producingphotoconductive substance according to the present invention isexplained. A primary sintering step for heating at a temperature of800°-840° C. for producing a solid phase reaction of starting materialcompound as described in the general chemical formula Bi₂ -(Sr₂Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y) -O_(z) where 0.4≦x≦1, y=2 and z=9-10.5,for 8-15 hours and annealing for 8-15 hours and a secondary sinteringstep after forming under pressure, heating at 900°-940° C. for 8-15hours and annealing at 100°-150° C./H are necessary steps for completingthe solid phase reaction and obtaining a uniform solid phase. Heating ata temperature higher than 1000° C. is not preferable because it ismelting. Moreover, heating at less than 900° C. cannot attain an objectof completing the solid phase reaction and it is not preferable.

Once such a type of superconductive optoelectronic device is formed withthe Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y) -O_(z), it must benaturally straightforward to further develop the new field from such adevice to other devices; and eventually to superconductiveoptoelectronic apparatus with the Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x)-Cu_(y) O_(z) system, for instance a switching device with no powerloss, an optically operating device with no power loss, an opticallyoperating logical device, a space parallel type optically operatingdevice, a camera or an image forming device possibly withsuperconducting wiring, a high-speed optically operating apparatus to bedriven at an extremely low power with higher optical efficiency, and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to theaccompanying drawings, in which:

FIG. 1 enumerates experimental results on the variation of powder X-raydiffraction pattern over x to clarify crystalline structure ofsuperconductive conjugate photoconductive substance in the Bi₂ -(Sr₂Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu₂ -O_(z) system;

FIGS. 2A, 2B and 2C are the schematic diagrams of circuit and timesequence for the repetitive measurement of pulse photoconductivity byusing blocking electrodes;

FIGS. 3A, 3B and 3C are the sectional view of a microwave SQUID and theblocking diagrams for the measurement of static magnetization;

FIG. 4A indicates characteristic data of wavelength dependence ofphotoconductive response Q(λ,T) of the basic substance Bi₂ O₃, and FIG.4B indicates characteristic data of a wavelength dependence ofphotoconductive response Q(λ,T) of a specimen of Bi₂ La₂ YCu₂ O_(z) ;

FIG. 5A is a characteristic plotting to exemplify the relation betweentemperature and photoconductive response Q(λ,T) of the basic substanceBi₂ O₃ (#B03). FIG. 5B is a characteristic graph showing the relationbetween temperature and photoconductive response Q(λ,T) of the basicsubstance Bi₂ O₃ :M²⁺ (#S213). FIG. 5C is a characteristic graph showingthe relation between temperature and photoconductive response of Bi₂(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) Cu₂ O_(z) for x=1 as photoconductivesubstance, and FIG. 5D is a graph showing the relation betweentemperature and resistance in the dark of the Bi₂ (Sr₂ Ca₁)_(1-x) (La₂Y₁)_(x) Cu₂ O_(z) for x=0 as a superconductive substance;

FIG. 6 displays characteristic plottings to indicate the temperaturedependence of dark resistivity ρ(T) (mΩ·cm) in the region of x=0-0.4 ofthe Bi₂ (Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) Cu₂ O_(z) system;

FIG. 7 displays characteristic plottings to indicate the temperaturedependence of photoconductive response Q(T,λ) in the region of x=0.9-1.0of the Bi₂ (Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) Cu₂ O_(z) system;

FIG. 8 is a quasi-phase diagram to exhibit the emergence or steptemperature Tps of photoconductivity and superconductive transitiontemperature Tsc of the Bi₂ (Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) Cu₂ O_(z) systemas a function of x;

FIG. 9 is a similar quasi-phase diagram to exhibit the superconductivetransition temperature Tsc and the emergence or step temperature Tps ofphotoconductivity of the Bi₂ -Sr₂ -Ca_(1-x) Y_(x) -Cu₂ -O_(z) system asa function of x;

FIG. 10A is a schematic diagram of the state density N(E) as a functionof energy E of the Bi₂ (Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) Cu₂ O_(z) system inthe case of x=1, and FIG. 10B is a schematic diagram of the statedensity N(E) as a function of energy E of the Bi₂ (Sr₂ Ca₁)_(1-x) (La₂Y₁)_(x) Cu₂ O_(z) system in the case of x=0;

FIG. 11 is a schematic cross section to exemplify an embodiment of theconstruction of a superconductive optoelectronic element according tothe present invention;

FIG. 12 is a schematic diagram to display an embodiment of theconstruction of the superconductive optoelectronic device according tothe present invention; and

FIGS. 13A and 13B are a schematic diagram to illustrate the constructionof the spatial parallel operation device with the use of thesuperconductive optoelectronic element alley according to the presentinvention.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS

The greater part hitherto known of the Ba-Pb-Bi-O, La-Cu-O, Y-Ba-Cu-Oand Bi-Sr-Ca-Cu-O systems of oxide compounds are usually insulators orsemiconductors in the ground state, that is, in the dark (i.e. in a darkplace condition), particularly at low temperature. Therefore, it ispossible to create an elementary excitation by giving a suitable energywith a suitable kinetic momentum above the ground state of thesesubstances. It has been presumed that an elementary excitation exceedingan energy gap merely breaks the ground state of a superconductor in theBCS theory. However, an insulative semiconductor has a possibility ofcreating an elementary excitation in coherent state in the conductionand/or valence bands such as a bipolaron and an exciton above the groundstate even in a thermally non-equilibrium state. These studies have beenmade in parallel with a study of a superconductor of a high criticaltemperature Tc. Apart from a trend of the study, however, the presentinvention has been completed by finding a superconductive conjugatephotoconductive substance correlative with a superconductive substanceto elicit the photoconductivity Q(λ,T) at photoexcitation of thespecified range of optical wavelength λat temperature below a criticaltemperature Tsc outside of the composition of superconductors. This is anew finding in the fields of fundamental physics and applied physicsfrom a novel point of view, that is, from the viewpoint of an elementaryexcitation concept.

In the present invention, the reason why the composition ofsuperconductive conjugate oxide photoconductive substance is limited tothe general chemical formula

    Bi.sub.2 -(Sr.sub.2 Ca.sub.1).sub.1-x (La.sub.2 Y.sub.1).sub.x -Cu.sub.y -O.sub.z

where 0.4≦x≦1, y=2, and z=9-10.5, is because since the composition ofx=0-0.3 is the condition to be a superconductor, so that the compositionof superconductor range x=0-0.3 as shown in FIG. 6 is excluded. Theinventor studied and examined with the region of substance of 0.4≦x≦1having a composition close to a superconductor and having a temperaturedependence of photoconductivity conjugate with superconductivity withina range where the substance does not become a superconductor. Theinventor discovered a fact that the substance of an insulator or asemiconductor for 0.4≦x≦1.0 in the dark reveals photoconductivity havinga temperature dependence in parallel to or conjugate withsuperconductivity at photoexcitation in a specified wavelength range of420-670 nm.

The study of the present invention has been initiated on the bases of afact that, since even Bi₂ O₃ has photoconductivity to visible light ofspecified wavelength, Bi₂ O₃ was recognized as the basic substance.Thus, the inventor examined whether any photoconductive substanceconjugate with superconductivity is obtained by adding additionalelement, and further examined a particular system adding Ca, Sr with Cuand the like to Bi₂ O₃. As a result, the inventor found a system ofphotoconductive substance consisting of the composition having the abovegeneral chemical formula which composition is close to but outside of asuperconductor and inherently an insulator or a semiconductor in thedark and having photoconductivity Q(λ,T) conjugate withsuperconductivity at a temperature (T) of less than the criticaltemperature of the superconductor at photoexcitation in a specifiedwavelength range λ.

The range of 0.4≦x≦1, y=2, and z=9-10.5 in the above general chemicalformula Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y) O_(z) is a compositionfor condition of photoconductive substance conjugate withsuperconductivity of the present invention. Here, when x=1, Bi₂ -(La₂Y₁)₁ -Cu_(y) O_(z), and this composition is most suitable for thecondition of the present invention.

An embodiment of such photoconductive substance is described. Thepresent inventor has studied a series of specimens in the Bi₂ (Sr₂Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y) O_(z) system where 0.4≦x≦1, y=2, andz=9-10.5, particularly a variation of the step temperatures T_(ps) inQ(T,λ) and T_(sc) in ρ(T) over x, namely, an influence of thecomposition of lanthanum (La) and yttrium (Y) to form a quasi-phasediagram. Here, the present inventor has performed a systematic study notonly of a superconductive phase but also of a semiconductor phase or aninsulator phase of the said substance. A large number of specimens ofBi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y) O_(z) system were preparedfrom the powder of CaCO₃, SrCO₃, Bi₂ O₃, CuO, La₂ O₃ and Y₂ O₃. Thecompositions x and y of the starting material were thoroughly examined,and here, it became clear that x can be controlled particularly at thecomposition of y=2. An oxygen content z can also be controlled to someextent by controlling a secondary sintering temperature and a coolingrate. Specimen No. S235 (x=1) was prepared by mixing 1.314 g of Bi₂ O₃,0.449 g of CuO, 0.918 g of La₂ O₃ and 0.318 g of Y₂ O₃ and firing themixture to be the formula Bi₂ La₂ Y₁ Cu₂ O_(z). Specimen No. S228(x=0.1) was prepared by mixing 1.329 g of Bi₂ O₃, 0.758 g of SrCO₃,0.257 g of CaCO₃ and 0.453 g of CuO, 0.094 g of La₂ O₃ and 0.033 g of Y₂O₃ and firing the mixture to be the formula Bi₂ (Sr₂ Ca₁)₀.9 (La₂ Y₁)₀.1Cu₂ O_(z), where z shows an oxygen amount, and z changes to z=9-10.5 bycontrolling the firing temperature and cooling rate, thereby differingphysical properties of a product obtained.

In the present invention, raw materials were compounded according to acompounding composition ratio, thoroughly stirred, ground, thereafterprimarily sintered at 800°-840° C., preferably 820° C. for 8-15 hourspreferably over 10 hours to carry out a solid phase reaction, annealedfor 8-15 hours, and thereafter the resulting product was used forpreparing pellets by forming under pressure. Moreover, these pelletswere secondarily sintered at 900°-940° C., preferably 920° C. for 8-15hours, more preferably over 10 hours, and annealed to room temperatureat 100°-200° C./H. In this manner, the former (x=1) reveals asuperconductive conjugate photoconductive phase of 80K class, while thelatter (x=0.1) reveals a superconductive phase of 80˜110K class.

An embodiment of preparing the similar specimens with the othercompositions is as shown in Table 1. Moreover, the informations of theircrystal structures are displayed with performing several X-ray analysesin FIG. 1.

                                      TABLE 1                                     __________________________________________________________________________    Bi.sub.2 (Sr.sub.2 Ca.sub.1).sub.1-x (La.sub.2 Y.sub.1).sub.z Cu.sub.2        O.sub.z                                                                                                         Primary sintering                                                                           Secondary sintering                                                      temper-       temper-              Speci- Bi.sub.2 O.sub.3                                                                  SrCO.sub.3                                                                        CaCO     La.sub.2 O.sub.3                                                                        time     ature                                                                              time     ture                 men    (6N)                                                                              (4.5N)                                                                            (3N)                                                                              CuO(3N)                                                                            (4N)                                                                              Y.sub.2 O.sub.3 (5N)                                                                (hr)     (°C.)                                                                       (hr)     (°C.)         x  No. (g) (g) (g) (g)  (g) (g)   (A)                                                                              (B)                                                                              (C)                                                                              (D)  (A)                                                                              (B)                                                                              (C)                                                                              (T)                  __________________________________________________________________________    0.10                                                                             S-228                                                                             1.329                                                                             0.758                                                                             0.257                                                                             0.453                                                                              0.094                                                                             0.033 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                0.20                                                                             S-229                                                                             1.360                                                                             0.690                                                                             0.234                                                                             0.464                                                                              0.190                                                                             0.066 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                0.30                                                                             S-230                                                                             1.351                                                                             0.599                                                                             0.203                                                                             0.461                                                                              0.283                                                                             0.098 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                0.40                                                                             S-231                                                                             1.347                                                                             0.512                                                                             0.178                                                                             0.460                                                                              0.376                                                                             0.131 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                0.50                                                                             Y-4 1.118                                                                             0.354                                                                             0.120                                                                             0.382                                                                              0.391                                                                             0.1.35                                                                              2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (290)                0.60                                                                             S-233                                                                             1.336                                                                             0.338                                                                             0.115                                                                             0.456                                                                              0.561                                                                             0.195 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                0.80                                                                             S-234                                                                             1.325                                                                             0.167                                                                             0.056                                                                             0.452                                                                              0.741                                                                             0.256 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                0.90                                                                             S-237                                                                             1.319                                                                             0.084                                                                             0.028                                                                             0.450                                                                              0.829                                                                             0.288 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                0.95                                                                             S-238                                                                             1.316                                                                             0.041                                                                             0.014                                                                             0.450                                                                              0.875                                                                             0.302 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                1.00                                                                             S-235                                                                             1.314                                                                             0   0   0.449                                                                              0.918                                                                             0.318 2, 10,                                                                              10 (820)                                                                              2, 10,                                                                              8  (920)                __________________________________________________________________________     Note)                                                                         Both primary sintering and secondary sintering were carried out in air.       Moreover, indication of A, B, C and D shows the step of heating A (hour),     increasing a temperature from room temperature to T (°C.), keeping     the temperature for B (hour), and thereafter lowering the temperature to      room temperature by taking C (hour).                                     

Experimental Method

A phase diagram of the Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y) O_(z)system oxide compound is a seven-element system, which is not yetcomplete at a preliminary stage. Particularly important is control of zfor the oxygen deficiency corresponding to a set of composition ratiosof x and y. In spite of many scientists' remarkable efforts, it willtake some more time to completes it. The inventor has been interested innot only a superconductive phase but also a photoconductivity in asemiconductor phase and an insulator phase in the dark. Many specimensof oxide compound in the Bi-(SrCa)(LaY)-Cu-O system were prepared frompowder of Bi₂ O₃, SrCO₃, CaCO₃, CuO, La₂ O₃ and Y₂ O₃. The inventor hasstudied the composition of material, the annealing and quenchingprocesses and the like in detail, and can control to some extent foroxygen deficiency.

Since specimens of oxide compound in the Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂Y₁)_(x) -Cu_(y) -O_(z) system are highly insulating at certain values ofx,y and z or semiconductive at least at low temperature which arecorrelative with or conjugate with superconductivity, the inventoradopted two types of techniques for resistivity and/or conductivitymeasurement in experiment. First, it turned out that the fast pulsetechnique (see FIG. 2A) with blocking electrodes overcomes severaldifficult problems in the measurement of an insulating specimen (ρ≧10⁸Ω·cm), such as Specimen No. S235 at temperatures down to 4.2K from 300K.Moreover, an electrode arrangement of lateral mode was employed, ifcircumstances require (see FIG. 2B). In measurement, an electric fieldpulse E was sustained at a certain value up to E≈5 KV/cm with 10 msecduration in a repetition rate of 13 Hz. Photoexcitation by using a dyelaser pulse of 3 nsec in width was synchronized at a suitable timewithin the time duration of applied electric field pulse (see FIG. 2C).

Second, for a specimen having proper conductivity (ρ≦10¹ Ω·cm) such asSpecimen No. S231, the inventor adopted a usual four probe method in theresistance measurement in the dark without photoexcitation (e.g.installed at a specimen holder in a cryostat).

Static magnetic susceptibility or magnitude of magnetization M (T,H) canbe measured at a weak field up to H≈500 Oe by using a microwave SQUID at9 GHz band. Characteristic features of this measurement system areseparately described (see FIGS. 3A, 3B and 3C).

In the case of photoconductivity measurement, a specimen wasphotoexcited at a wavelength range of λ=420-470 nm with the use of apulsed dye laser. Spectral response was carefully examined by payingspecial attention. A number of excited photocarriers is of the order of10⁶ ˜10⁸ but the density can be 10¹² Ω/cm³ within a thin layer of 10⁻³-10⁻⁴ cm in the vicinity of a surface when an absorption coefficient islarger. Photoconductivity signals were detected in a synchronized modewith the use of a Boxcar integrator.

Experimental Result

A specimen of Bi₂ (Sr₂ Ca₁)₁ Cu₂ O_(z) (x=0) such as Specimen No. S182looks black, and resistance at room temperature is usually of the orderof ρ≦10⁻³ Ω·cm. According to the inventor's observation, when repetitivepulse technique is applied to Specimen No. S235 (x=1, insulator), asignal of photoconductivity emerges at and grows below 80-110K or40-60K. Origins of these emergences are probably different each other.

First, the dependence of photoconductivity Q(λ,T,E) on the appliedelectric field E is almost linear up to E≈4 kV/cm at T=4.2K. FIG. 4A isthe typical spectra response of pulse photoconductivity Q(λ,T) ofSpecimen No. B03 of Bi₂ O₃, and FIG. 4B is the typical spectra responseof pulse photoconductivity Q(λ,T) of Specimen No. S235 (x=1) of Bi₂ -La₂Y₁ -Cu₂ -O_(z) over a wavelength region of λ≈420-670 nm. In thisconnection, FIG. 4A is a new datum of photoconductivity spectracorresponding to light absorption of Bi₂ O₃ previously observed first bythe inventor throughout the world and should be used as a standard.

Secondly, the temperature dependences of photoconductivity Q(λ,T) at thewavelength range λ=420-680 nm were examined for Specimen No. B03 of Bi₂O₃ as shown in FIG. 5A, for Specimen No. S213 of Bi₂ O₃ :M²⁺, as shownin FIG. 5B and for Specimen No. S195 of an insulator Bi₂ (La₂ Y₁)Cu₂O_(z) as displayed in FIG. 5C. It is surprising to recognize thatconspicuous similarities definitely exist among general characteristicsof mutually interrelated photoconductivities Q(λ,T) of Specimen Nos.B03, S213 and Specimen Nos. S195, S235 (x=1 in FIG. 7). No one can failto recognize that "photoconductive response Q(λ,T)" in specimens of aninsulator or a semiconductor emerges at an absolute temperature below80-110K and less than 40-60K together with the lowering of a temperatureand monotonously increases and thereafter further increases at atemperature of less than 10K, as if superconductivity latentlyunderlies.

Actually, FIG. 5D exemplified the dark resistivity λ(T) of Specimen No.S182 of superconductor Bi₂ (Sr₂ Ca₁)₁ Cu₂ -O_(z) as a function oftemperature. One can immediately note that Specimen No. S182 becomessuperconductor at and below T=80-110K and T=65-85K. With a slight shiftin T, the photoconductive response Q(λ,T) displayed in FIG. 5C has asurprisingly well one to one correspondence thereto.

FIGS. 4 and 5 only illustrate the cases for x=0 and x=1 as the bothextrem ends of the Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu₂ -O_(z) systemfor the sake of definiteness. In general, the condition is rathercomplicated in 0<x<1.

Specimen No. S182 (x=0) of Bi₂ Sr₂ CaCu₂ O_(z) is a knownsuperconductor. When the composition is changed from x=0 to x=1 in theorder shown in Table 1, the crystalline structure varies as shown inFIG. 1, but it becomes rather simpler in S235 (x=1).

On the other hand, the temperature dependence of resistance ρ(T) ofthese series in the dark drastically varies as shown in FIG. 6. As thecomposition x increases from x=0 to x=0.3, the absolute value of ρ(T)becomes larger and the superconductive critical temperature Tscsimultaneously becomes lower. Further increase of x in the compositiondoes convert the specimen into a semiconductive phase at 0.3<x<0.4. Whenx is even further increased, the value of ρ becomes larger, thematerials in that composition become insulators. Eventually, it becomesextremely difficult to measure resistance in the dark by a usualfour-probes method. This difficulty in the measurement results fromconditions peculiar to high resistance substance, such as non-ohmicproperties of contact electrode, formation of space charge and tinysignal to noise (S/N) ratio due to a low concentration of carriers.

Therefore, for a conductivity measurement of specimens in these regions,one has to adopt the transient technique of pulsed photoconductivitymeasurement with blocking electrodes, the principle of which isexplained in FIGS. 2A and 2B. This method is effective to a measurementof rather high impedance materials. Actually, as shown in FIG. 7 in caseof the specimens for 0.9≦x1, the photoconductivity signal Q(T,λ) atphotoexcitation with wavelength λ=476˜500 nm becomes observable at andbelow a certain temperature Tpc.

It should be noted here that the value of Tsc once decreases as xincreases from x=0 to x=0.3 in the superconductive region, and revealingphotoconductivity after superconductor insulator transition, the valueof Tpc increases again in the region from x=0.9 to x=1.0. Thesesituations are illustrated in FIG. 8 in a form of a phase diagram likescheme for the Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu₂ O_(z) system.

FIG. 9 displays a similar diagram for the Bi₂ -Sr₂ (Ca₁)_(1-x) Y_(x)-Cu₂ -O_(z) system in the same manner as in FIG. 8. In this case, it isknown that the lattice spacing continuously varies by x, but thecrystalline structure never changes. Photoconductivity emerges only inthe vicinity of x=0.

In any case, both superconductive and photoconductive regions haveseveral values of the critical temperatures Tsc and TPC or the steptemperatures Tps, respectively. The values of Tsc and Tps, vary with x.They slightly shift but correspond to each other across their transitionregions.

It is not easy to simply understand these experimental facts. It mustturn out that a heating effect of the specimen by photoexcitation issufficiently small when if we carefully examine and estimate the effect.Specimen Nos. S195 and 235 of Bi₂ -La₂ Y₁ -Cu₂ -O_(z) (x=1) are asemiconductor or rather an insulator even at T=300K. However, one canmainly conceive that "photoconductivity" observed by using the transienttechnique with the arrangement of blocking electrodes and"superconductivity" in Specimen No. S182 of a superconductor areprofoundly correlated. As illustrated in FIGS. 4A, 4B and FIGS. 5A, 5B,5C and 5D, this is probably, due to a potentiality of the insulatorportions within the specimen to be convertible to superconductor bydoping. However, surprising is an existence of such fact that even in aninsulator Specimen No. S195, there is an "emergence of superconductiveconjugate photoconductive phenomenon" to reveal an implicit correlationas if superconductivity latently underlies.

Studies of Experimental Results and Further Discussions

A specimen of the Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y) O_(z) systemin the semiconductor or insulator region is usually gray in color. Aphotoconductive spectral response Q(λ,T) shown in FIGS. 4A and 4Bsuggests that there exists a region similar to Bi₂ O₃ even notnecessarily in atomic layers but to some extent within the inside of thespecimens of the Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -O_(z) system.

Optical absorption and photoconductivity of Bi203 itself have not beensufficiently clarified yet even by an experiment or an exciton theory.However, an exciton here is considered to be a typical example ofFrenkel exciton due to charge transfer within a cation shell andneighboring anion cells. The position of fine structure inphotoconductivity Q(λ,T) of the abovementioned Bi-(SrCa)(LaY)-Cu-Osystem reasonably corresponds to the fundamental absorption edgestructures of the basic substance Bi₂ O₃. One recognizes several finestructures to be considered conspicuous due to excitons. For instance,the spectra of photoconductive response of Bi₂ -La₂ Y₁ -Cu₂ -O_(z) aresimilar to that of the reference substance Bi₂ O₃. In the vicinity ofλ=568-580 nm in this spectra, we recognize a structure which isconsidered to correspond to the n=2 state in an exciton series of Bi₂O₃. Namely, there exists a phase similar to Bi₂ O₃ at least in a finiteproportion in the substance of the Bi-(SrCa)(LaY)-Cu-O system, which noone can ignore. Crystalline structures are slightly different from eachother, but photoexcited conduction electrons and the holes aredissociated and definitely mobile (see FIG. 10A).

A conduction electron and a hole in the standard type Bi₂ O₃ crystal areconsidered to form a rather "small polaron" in terms of the couplingconstant α. In any case, "an emergence of photoconductivity Q(λ,T)" ininsulating specimen clearly relates to "an emergence ofsuperconductivity", as if superconductivity is latently conjugate with aphotoconductive phenomenon. Therefore, the polaron effect is at leastpotentially of remarkably importance as shown in FIGS. 4A, 4B and FIGS.5A-D, whether it is a "large polaron" based on the interaction with LO(longitudinal optical type) phonon or a "small polaron" due to theJahn-Teller effect or an intermediate coupling region based on botheffects as well as the "polaron effect due to electronic polarization".Dynamical polaron effects are considered to be effective in a coherentlyhybridized form of elementary excitations. It is necessary to payspecial attention to polarons due to electronic polarization, which arealso referred to as "excitonic polarons". By examining theseexperimental results, we recognize a close relation between polarons andexcitons.

As shown in FIG. 10A, these polarons and excitons had yielded out of theoptical interband transition from the hybridized valence band state ofO(2p) and Bi(6s) or Bi(6p) conduction band (possibly mixed with theCu(4s,3d) (not shown) depending on case) leaving a hole (white circle)in the O(2p)⁶ Bi(6s)¹ state with LO phonon interaction. However, asshown in FIG. 10B (x=0), a polaron in the Bi-(SrCa)(LaY)-Cu-O can becreated not by optical excitation but by substitution of (La₂ Y₁) by Sr₂Ca₁. Here, FIG. 10B shows the case of a superconductor with x=0 withintraband exiton, which already has been known. There has recently beenproposed that the hybridized valence electron state is caused byO(2p)Bi(6p), and the conduction band is caused by Bi(6d). Situationhere, however, remains without substantial change.

Holes in the hybridized bands of O(2p) and Bi(6s) in Bi₂ -(Sr₂Ca₁)_(1-x) (LaY)_(x) -Cu_(y) -O_(z) can be created from the ground stateof a many-body system either by an interband optical transition or bydoping additional element together with interband excitation. But, here,a correlation effect between electrons is extremely important in anycase. One must pay serious attention not only to the dynamical valencefluctuation between Bi³⁺ and Bi⁵⁺ and between Cu²⁺ and Cu³⁺, but alsofurther to the dynamical valence fluctuation between Cu¹⁺ and Cu²⁺,particularly between Bi³⁺ and Bi⁴⁺. Therefore, to clarify the mechanismof high-Tc superconductivity, there exists a sufficient reason toconsider a potential role of an ensemble of polarons, whether large orsmall, particularly an ensemble of polarons closely associated withexcitons. The ensemble of the united polarons and excitons here isconsidered to be a set of bipolarons and polaronic excitons and/orexcitonic polarons due to the dynamical electron-phonon interaction andthe dynamical electron correlation, namely, "exciton-mediatedbipolaron". As shown in FIG. 4B, it was confirmed that thephotoconductive responses Q(λ,T) of the Bi-(SrCa)(LaY)-Cu-O system havewavelength dependence in the region of 420-670 nm similar to thephotoconductive spectra of the basic substance Bi₂ O₃ shown in FIG. 4A.Therefore, by studying the elementary excitations, we can approach toclarify the nature of the superconductive ground state, irrespective ofan enormous difference in the carrier densities. To our knowledge, thepresent invention based on these experimental results is the firstexperimental confirmation that the Bi-(SrCa)(LaY)-Cu-O oxide systemconsists of superconductive conjugate photoconductive substance withreal superconductors. It was experimentally and clearly confirmed thatthe mechanism due to the polarons and excitons underlies inherentlycommonly over the present substances and the oxide-series hightemperature superconductor.

In case of studying the physical properties of the Bi-(SrCa)(LaY)-Cu-Osuperconductive photoconductive substance according to the presentinvention, the inventor has found that the critical temperatures115K-105K (high Tc phase) and 85-65K (low Tc phase) for startingsuperconductivity expected in the known superconductor well correspondto the temperature for revealing the superconductivity and thetemperature for revealing photoresponse in the superconductive conjugatephotoconductive substance according to the present invention.

The inventor has found for the first time that the Bi-(SrCa)(LaY)-Cu-Ooxide superconductive photoconductive substance has profound correlationto "superconductivity" at x=0 and "conjugate photoconductivity" in theproximity of x=1 by the above selection of x. Therefore, the inventorreconfirmed the existence of the dynamical mechanism due to polaron andexciton in high temperature superconductivity, that is, the dynamicalmechanism due to "exciton-mediated bipolarons".

By these studies and discussions, the inventor proposes severalpossibilities in technological aspects, FIG. 11 is a schematic crosssection to exemplify a constructed form of a superconductiveoptoelectronic element according to the present invention. In thepresent embodiment, we can explain the case to devise the element as asuperconductive phototransistor (VG≠0). On the substrate 1, e.g., madeof SrTiO₃, is formed a photoconductive gate region 2. The gate region 2comprises a superconductive-conjugate photoconductive Bi-(SrCa)(LaY)CuOlayer of 0.2 μm-1.0 mm in width and 1-10 μm in thickness. ThisBi-(SrCa)(LaY)CuO layer provides special photoconductivity atphotoexcitation in the wavelength region of 420-670 nm. at and below acritical temperature of 105-115K and 65-85K of a certain superconductivematerial consisting of Bi-(SrCa)(LaY)CuO. On both sides of the gateregion 2 are formed a source region 3 and a drain region 4. These sourceregion 3 and drain region 4 are composed of a Ba(SrCa)(LaY)CuOsuperconductive layer showing superconductivity at and below a criticaltemperature of 105-115(K) and 65-85K. Moreover, on the gate region 2,the source region 3 and the drain region 4 is formed an SiO₂ layer 5 of1 μm in thickness with optically transparent and electrically insulatingproperties, and a NESA glass layer 6 with bias electrodes is formedthereon. Between the bias electrode on the NESA glass layer and thesource region 3 is connected a bias source V_(G) and between the sourceregion 3 and the drain region 4 are connected a bias source V_(SD) andan output resistance R. In addition, it is possible to construct theregions 3, 4 of superconductive Ba(SrCa)(LaY)CuO system from thephotoconductive Ba(SrCa)(LaY)CuO region 2 by continuously varying thecomposition x of (LAY) in the Ba-(SrCa)(LaY)CuO superconductivephotoconductive substance from x=1 to 0.4 and to x=0.

When such a superconductive optoelectronic element prepared via theabove process of construction is cooled down to and below the criticaltemperatures of 105-115(K) of a material layer of Ba-(SrCa)(LaY)CuO andthe temperature below 65-85K with an incident light of an excitationwavelength range, photocarriers density in proportion to the intensityof incident light are realized in the gate region 2. The photocarriersaccelerated by the bias V_(SD) between the source and drain yield acurrent and result in an output voltage across the output resistance R.Moreover, the density of photocarriers is controlled via the lightintensity and the bias source V_(G), so that the bias source V_(G) canbe set appropriately in accordance with a purpose. With the aboveconstruction, it is possible to obtain an output characteristics inaccordance with the incident light intensity, so as to realize asuperconductive optical switching element. The source region and thedrain region are particularly made of superconductive material, so thata substantially new superconductive optoelectronic element can berealized without heat dissipation during operation.

FIG. 12 is a schematic diagram showing an embodiment of integrating thesuperconductive optoelectronic elements shown in FIG. 11 in the form ofan alley. When the superconductive optoelectronic elements according tothe present invention are integrated at high density in the form of onedimensional or two dimensional alley, it is possible to materialize adevice like a camera by minimizing heat dissipation during operationwith appropriate superconductive wirings among various elements as abackground. It is also possible to materialize the main portions forsignal detection in an optical computer for performing a spatiallyparallel operation. There is also a possibility of multi-channeloperation by selecting the wavelength of an incident light source used.

FIGS. 13A and 13B schematically illustrate an embodiment of opticaloperation in the projectioncorrelation optical system of the spatiallyparallel optical computer [see T. Yatagai: OYO BUTSURI (Applied Physicsin Japanese), 57 (1988) p. 1136] with the use of superconductiveoptoelectronic elements according to the present invention. A pluralityof optical signals made in parallel from an alley-like light source 10are projected onto an encoded image mask pattern 11. The image maskpattern 11 carries encoded image information in a mask fashion. Aplurality of light beams passed through the encoded image mask pattern11 are incident in parallel to each element corresponding to a compositemask optical element alley 13 via a correlation image screen 12. Sincean encoded signal modulated by the mask screen is formed in each opticalelement, an operation result is obtained from photoelectric outputsignal from each optical element. If each element of the optical elementalley 13 is constructed with the superconductive optoelectronic elementaccording to the present invention, it is possible to carry out aparallel optical operation under the condition of minimizing heatdissipation during the operation.

The embodiment described above represents the three-terminal element asan example, but a two-terminal element also can be realized. Thus, thephotocarrier created at V_(G) =0 may be influenced by a superconductiveproximity effect irrespective of a small coherence length viasuperconductive photoconductivity, so that the superconductiveoptoelectronic element can be served as a superconductive Josephsonjunction element based on light irradiation. Such a two-terminal elementcan hold a position as "superconductive photoconductive or opticallycontrolled Josephson junction device". In this case, it is necessary toappropriately select gate width and incident light amount.

It is possible to arrive at the following conclusion from these results.As a result of extensive studies, by applying not only the D.C. 4-probemethod but also method of the repetitive pulse photoconductivitymeasurement for studying transport phenomena in the temperature range ofT=4.2K-300K, and by using the microwave SQUID for static magnetizationmeasurement, the inventor confirmed that "photoconductivity" is closelycorrelated and conjugated with "superconductivity (zero resistance anddiamagnetism)" and invented "the superconductive conjugatephotoconductive substance Bi₂ -(Sr₂ Ca₁)_(1-x) (La₂ Y₁)_(x) -Cu_(y)-O_(z) system", 0.4≦x≦1, y=2 and z=9-10.5, and also invented a methodfor producing the same. Besides, the inventor invented a superconductiveoptoelectronic element and device by using the same. The presentinvention has been developed in parallel with such theoreticalconsideration that "dynamical mechanism due to polaron and excitons",namely, the mechanism due to "exciton-mediated bipolarons", is proposedfor "the high temperature superconductivity", and these new materialswill develop the up-to-date scientific technical field of"superconductive optoelectronics" which directly controlssuperconductivity by light.

Although the invention has been described with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that numerous change in details may beresorted to without departing from the scope of the invention ashereinafter claimed.

What is claimed is:
 1. A superconductive-conjugate photoconductivesubstance of a Bi-SrCa(LaY)-Cu-O oxide with a composition having ageneral chemical formula of

    Bi.sub.2 -(Sr.sub.2 Ca.sub.1).sub.1-x (La.sub.2 Y.sub.1).sub.x -Cu.sub.y -O.sub.z,

where 0.4≦x≦1, y=2 and z=9-10.5, wherein, when said substance is at atemperature below the critical temperature of a Bi-SrCa(LaY)-Cu-Osuperconductor, said substance is (A) a conductor when exposed to light,said conductor having a photoconductivity Q(λ,T) conjugate withsuperconductivity of the Bi-SrCa(LaY)-Cu-O superconductor atphotoexcitation in an optical wavelength range λ) of 420-670 nm, and (B)an insulator or a semiconductor when not exposed to light.